256 MICROBIOLOGY OF SOIL. 



It appears, therefore, that ammonia production in soils rises or falls 

 as the rainfall or irrigation is increased or decreased, or as the soil water 

 is more or less thoroughly conserved by proper methods of tillage. In the 

 same way, seasons of high temperature favor ammonification while 

 seasons of low temperatures discourage it. This point is well illustrated 

 by the observations of Marchal that at o to 5 only traces of ammonia 

 were formed in his culture solutions; that at 20 ammonia production 

 was quite marked, and that at 30 the maximum was reached. Moreover, 

 apart from the seasonal variations in any one locality, there is a wide 

 range in ammonia production, as we pass from the torrid to the temperate 

 and from the latter to the frigid zones. 



Species and Numbers. Ammonia production is a function common 

 to most soil bacteria. Already in the earlier experiments of Marchal, 17 

 out of the 31 species tested were found capable of producing ammonia. 

 Prominent among these ammonifiers were B. mycoides, B. (Proteus) vul- 

 garis, B. mesentericus vulgattis, B. janthinus, and B. subtilis. Of a con- 

 siderable number of soil bacteria tested by Chester all but one were ob- 

 served to produce ammonia. In Gage 's experiments with sewage bacteria, 

 17 out of 20 species tested proved to be ammonifiers. Similarly, a num- 

 ber of species tested by the writer, among them B. coli, B. cholerae suis, 

 B. (Proteus) vulgaris, B. subtilis, B. tnegaterium, etc., all produced ammonia 

 in meat infusions. A mass of additional data, accumulated by different 

 investigators, furnish further proof that ammonia production is a common 

 function of soil bacteria. 



The more prominent ammonifiers, including members of the B. subtilis 

 group and certain Streptothrices, are numerically important in all arable 

 soils. Their numbers are affected, however, by the amount and compo- 

 sition of the soil humus. It has been found, for instance, that additions 

 of straw and of strawy manure increase markedly the numbers of B. 

 subtilis and of other members of the group. An increase in the numbers 

 of certain ammonifiers is caused also by additions of lime or of green 

 manure. For example, in experiments carried out by Lipman and 

 his associates portions of fertile soil inoculated with B. mycoides were 

 found to contain, a month later, 2,000,000 of bacteria per g. of soil. 

 In similar soil portions that had also received additions of grass the num- 

 ber was twice as great. 



Relative Efficiency of Different Species. In Marchal 's experiments 

 already referred to, the species employed showed marked differences in their 



